Numerous electrical and non-electrical power and communication systems are known for use in unbranched or multilateral oil and/or gas production wells.
U.S. Pat. Nos. 5,706,892; 5,706,896 and 5,721,538 disclose that a multilateral well may be equipped with a hardwired electrical or with a wireless communication system and that such a wireless system preferably transmits acoustic waves through a string of well tubulars such as the production tubing. Disadvantages of the known system are that installation of a wire tree in a multilateral well is a complex and expensive operation and that a wireless acoustic transmission system will suffer from high transmission losses and background noise. These disadvantages are particularly significant if the well is equipped with an expandable casing and/or production tubing. Around such an expanded well tubular there is hardly any or no annular space left for housing of the electrical cables and as a result of the physical contact between the expanded tubular and the surrounding formation acoustic signals will be dampened to a high extent.
Numerous other hardwired or wireless power transmission and communication systems are known, which have in common that they require complex and expensive equipment and that they are not suitable for use in multilateral wells.
U.S. Pat. No. 4,839,644 and European patent No. 295178 disclose a wireless communication system known as "Tucatran" which generates antenna currents in an unbranched well where the production tubing and surrounding well casing are electrically insulated from each other. The requirement of electrical insulation between the tubing and the casing is often difficult to accomplish in e.g. curved borehole sections and areas where brine is present in the tubing/casing annulus. International patent application WO80/00727 discloses another signal transmission system which utilizes an electrical circuit formed by a production tubing and a surrounding well casing.
U.S. Pat. No. 4,484,627, UK patent application No. 2322740 and International patent applications Nos. PCT/GB79/00158; PCT/GB93/01272 and PCT/EP96/00083 disclose other downhole electric transmission systems which utilize an externally insulated tubing in an unbranched well.
The present invention aims to overcome the disadvantages of the known transmission systems and to provide a downhole power and/or signal transmission system which can be used to transmit electrical power and/or signals throughout a multilateral well system in a safe and reliable manner even if the well comprises expandable well tubulars and without requiring complex wire trees or production tubing that are electrically insulated from the surrounding well casings.